The present invention relates to a device for disaggregating derived timber products according to the preamble of the main claim and to a method for implementing the same.
A device of this type has been disclosed (DE 198 19 988). In this device, a screw feeder functions as the transport device for conveying the disaggregation material through the impregnation and pre-swell shaft, while the function of the second screw feeder is to discharge the already disaggregated material from the disaggregation shaft. In the disaggregation shaft itself, the disaggregation material is not conveyed by screw feeders. The disadvantage of this device is the fact that, during disaggregation under pressure, appropriate measures must be taken to make the filling shaft pressure-sealable.
The object he present invention is thus to create a device and a method which permit continuous, and therefore simple and economical disaggregation of derived timber products.
This object is achieved by a device for disaggregating pieces of derived timber products made from cellulose-containing and/or lignocellulose-containing materials, specifically particle board, medium-density fiberboard and the like, with a transport device and at least one disaggregation vessel, characterized in that the disaggregation vessel and the transport device are designed as at least one combined-function plug screw feeder with an outer casing attached to the housing and a drivable shaft located therein provided with helices of a predefined pitch, the pieces of derived timber products being conveyed in the plug screw feeder from a feed opening in the outer casing to an end-face discharge opening in the axial direction (direction of conveyance), and characterized in that a delivery opening for steam is provided in the outer casing between the feed opening and the discharge opening.
The object is also achieved by a process for disaggregating pieces of derived timber products made from cellulose-containing and/or lignocellulose-containing materials, specifically particle board, medium-density fiberboard and other residual materials and waste produced from lignocellulose-containing materials, using a conveyance device as well as a disaggregation vessel, characterized in that the derived timber products are conveyed continuously through pressure-sealed, sequentially arranged plug screw feeders, thereby undergoing the action of shearing and frictional forces occurring in the plug screw feeder, and in that steam is forced into at least one of the plug screw feeders which is neither the first nor the last plug screw feeder.
The surprising determination was made that an arrangement of preferably at least three plug screw feeders in tandem permits the passage of the disaggregation material such that the feedstock is able to seal the plug screw feeders to the degree necessary for performing the disaggregation process under pressure. Here the steam is introduced, preferably at a pressure of 2 bars, into one of the plug screw feeders which is neither the first nor the last one in the sequence of plug screw feeders. Arrangements of, for example, five or more plug screw feeders are preferred, the steam being introduced into the middle section of the sequentially arranged plug screw feeders, for example, into the third in a sequence of five plug screw feeders. While in the side plug screw feeder[s] here, a pressure drop reaching atmospheric pressure may be observed, the arrangement results in the working pressure being sustained in the middle section, said pressure being up to 11 bars but at least 2 to 3 bars of overpressure.
A fundamental advantage of a method thus implemented on a completely continuous basis according to the invention lies specifically in the fact that complex and expensive measures to seal the system may be dispensed with and that continuous processing is nevertheless possible. Processing is thus possible without the opening and subsequent sealing of system components, and as a result there is no concomitant loss of energy or discharge of pollutants from the system. Thus from both the standpoint of economy and ecology, the device may thus operated at an optimal level.
In the method according to the invention, specifically when at least five plug screw feeders are used, disaggregation pressures of 2 bars to 11 bars of overpressure may be sustained, the range of operation being preferably between 3 bars and 8 bars, and especially between 2 bars and 6 bars. When steam is introduced at an overpressure of 4 bars, the disaggregation temperature at the steam inlet is approximately 143xc2x0 C.
Since passage through the screw feeder entails not only the conveyance of the disaggregation material but also its homogenization, the friction of the particles against each other results in a continuous comminution mechanically and facilitates chemical hydrolysis. The result is that the disaggregation of the particles is accelerated such that a residence time of only a few minutes is sufficient to produce a high level of disaggregation. In the event the derived timber products are more hydrolysis-resistant, appropriate means may be employed to accelerate hydrolysis such as raising or lowering the pH.
The disaggregation process from the addition of the comminuted particleboard fragments through the completely disaggregated, moist wood pulp is thus effected by multiple, at least three, sequentially arranged screw feeders. These pressure-sealed plug screw feeders perform multiple functions: conveyance as well as continuous comminution and hydrolysis of the pre-swollen wood pulp, initially by the friction of the particles against each other, and the actual disaggregation process under the action of water, steam and absorption of heat within the tubular screw feeder system. Use of a modular design is possible here in which multiple identical screw feeders and drive systems are employed, which use significantly reduces cost.
The independently controllable plug screw feeders and drive systems allow the feed rate to be adapted to existing process conditions, the maximum limits being set only by the dimensioning of the conveyance systems and the reaction speed of disaggregation.
Since the entire process proceeds under pressure, the entire system must be pressure-sealed, a feature which has the advantage of preventing any harmful emissions emanating from the disaggregation process from entering the environment.
Since the system is operated without generating effluents, environmental considerations have been accorded a high priority.
It has proven useful to fill the first plug screw feeder into which the disaggregation material has been introduced with preheated water. The water temperature here can range between approximately 20xc2x0 C. and 95xc2x0 C.
Conveyance by the screw feeder within the system allows for homogenization of the wood pulp, and the constant friction caused by the conveyance creates desirable and continuous comminution, thereby facilitating wetting of the surface. The result is that the enthalpy of the steam may be utilized over the entire length of the tube since the friction continuously separates the solubilized layers mechanically and exposes the underlying, not-yet-separated layers which are then readily acted upon by the steam. The process may be further accelerated by using higher process pressures.
Waste gas heat can be almost completely utilized and is thus not lost as is the case with the discontinuous process of the prior art. The heat provided by the moist disaggregated wood pulp is also utilized by the fact that the conveyance path from the point after the material has exited the last screw feeder to the drier is extremely short. This feature minimizes heat loss and drying cost.
The absorption of water, steam and heat by the disaggregation material may be very precisely adjusted by controlling the rate of feedxe2x80x94a feature which facilitates additional optimization of the process parameters, especially energy consumption, and optimal process control.
The plug screw feeders may be arranged sequentially in any desirable configuration. To save space, the preferred arrangement is one in which the outlet of each plug screw feeder discharges at right angles into the inlet section of the following plug screw feeder.
As the material is conveyed plug screw feeders are employed, according to the invention, which create a pressure in the conveyed material. These screw feeders, which are also designated plug screw feeders, are, for example, unilaterally bearing-mounted screw feeders, the outlet opening of which runs in the axial direction. They may have a progressive or degressive pitch, that is, spacing per turn between helices may be made larger or smaller.
The temperature of disaggregation has a significant effect on the hydrolysis of the derived timber products. The higher the pressure of the steam utilized for disaggregation, the higher is the disaggregation temperature. At a steam pressure of 4 bars overpressure, the temperature is approximately 143xc2x0 C. Disaggregation temperatures in the range of 8 bars may, depending on the type of wood and other process parameters, result in undesirable discoloration of the chips. Steam pressures must therefore be carefully monitored. Generally, pressures between 7 bars and 11 bars are selected, but preferably those between 2 bars and 8 bars, and especially in the range of between 3 bars and 6 bars of overpressure.
The rate of conveyance through the plug screw feeders varies as a function of the number of screw feeders employed. Given a greater number of screw feeders, the rate of throughput may be increased, whereas for a smaller number this rate must be reduced in order to ensure the required residence time for disaggregation. Depending on the pressure, complete disaggregation may be generally achieved after a residence time of around approximately 30 minutes, and preferably 20 minutes, such that the use of more than three screw feeders entails a residence time per plug screw feeder of only a few minutes. Compared to traditional discontinuous disaggregation processes, this amounts to a considerable reduction in disaggregation time.
Under the above conditions, the plug screw feeders generally attain rotational speeds in the range of a few rotations per minute, for example, 3 rpm to 10 rpm.
When the material is introduced into the first plug screw feeder, it has proven to be advantageous to add preheated water. Water temperatures may range between 200xc2x0 C. and 95xc2x0 C. Acting together with the charged wood materials, the water creates an additional seal against the external atmospheric pressure for the overpressure utilized for disaggregation.
The wood material to be disaggregated is precomminuted at a coarse level before disaggregation, particle sizes in the range of approximately 3 cm to 10 cm, preferably 3 cm to 5 cm, having proven to be especially suitable. Comminution to less than 2 cm brings no advantage since the removal of coatings or other impurities then becomes more difficult.
Experience has shown that, under favorable conditions, for example, with proper sealing of the individual plug screw feeders, only three screw feeders arranged sequentially are sufficient to sustain the appropriate disaggregation pressure matching the pressure of the introduced steam in the middle screw feeder. In an arrangement of more than three plug screw feeders, the zone of desirable reaction pressure is extended, with the result that the throughput rate for the individual plug screw feeders may be increased.
The first plug screw feeder is advantageously equipped on the material-charging side with a high-pressure lock. Pressure control is not necessarily required at the outlet side since the pressure here is allowed to fall to ambient pressure.